1. Field of the Invention
The present invention relates to a high frequency signal line and an electronic device, and more specifically relates to a high frequency signal line in which a signal line is provided on a element assembly including stacked insulator layers, and an electronic device.
2. Description of Related Art
In a high frequency signal line which has a triplate-type strip line structure in which a signal line is vertically sandwiched by ground conductors, the line width of the signal line is increased in order to reduce high frequency transmission loss of the signal line. Accordingly, the surface area of the signal line increases, and the area of a portion of the ground conductors facing on the signal line increases. As a result, the high frequency transmission loss of the signal line decreases. The high frequency transmission loss refers to a loss which is mainly caused by transition of a high frequency signal to heat in an impedance matched state.
However, as the line width of the signal line increases, the area of a region where the signal line and the ground conductors face each other increases, so that the electrostatic capacitance generated between the signal line and the ground conductors increases. Therefore, in order to make the characteristic impedance of the high frequency signal line equal to a predetermined characteristic impedance (e.g., 50Ω), it is necessary to increase the distance between the signal line and the ground conductors such that the electrostatic capacitance therebetween is reduced. However, as the distance between the signal line and the ground conductors increases, the thickness of the high frequency signal line increases.
In view of such a problem, a flexible substrate described in Japanese Laid-Open Publication No. 2007-123740 has been proposed. FIG. 16 is a view of a flexible substrate 600 described in Japanese Laid-Open Publication No. 2007-123740, which is viewed in plan from the layer-stacking direction.
The flexible substrate 600 includes a signal line 602 and a ground layer 604. The signal line 602 is a linear conductor. The ground layer 604 is stacked on the upper side of the signal line 602 in terms of the layer-stacking direction via a dielectric layer. Although not shown, another ground layer is provided on the lower side of the signal line 602 in terms of the layer-stacking direction. In the flexible substrate 600, the ground layer 604 has a plurality of openings 606. The openings 606 have a rectangular shape and are aligned in a row on the signal line 602 along the extending direction of the signal line 602. Therefore, when viewed in plan from the upper side of the layer-stacking direction, the signal line 602 partially overlaps the ground layer 604. As a result, the electrostatic capacitance generated between the signal line 602 and the ground layer 604 decreases. Thus, the distance between the signal line 602 and the ground layer 604 can be reduced, and the thickness of the flexible substrate 600 can be reduced.
However, the flexible substrate 600 described in Japanese Laid-Open Publication No. 2007-123740 has a problem in that reducing the thickness of the flexible substrate 600 is still difficult as described below. FIG. 17 is an equivalent circuit diagram of the signal line 602 and the ground layer 604.
In the flexible substrate 600, when a high frequency signal flows through the signal line 602, electric currents i1, i2 flow through bridge portions 608 lying between the openings 606 of the ground layer 604 due to electromagnetic induction as shown in FIG. 16. The electric currents i1, i2 flow in mutually opposite directions from a center of the bridge portions 608 in terms of the right-left direction. Here, the ground layer 604 of the flexible substrate 600 has a circuit configuration shown in FIG. 17. More specifically, the right half of the bridge portions 608 forms an inductor component L11, and the left half of the bridge portions 608 forms an inductor component L12. Also, a capacitor component C10 is formed between the signal line 602 and the ground layer 604. When the electric current i1 flows through the inductor component L11 and the electric current i2 flows through the inductor component L12, a magnetic field produced by the inductor component L11 and a magnetic field produced by the inductor component L12 cancel each other because the direction of the electric current i1 and the direction of the electric current i2 are opposite to each other. As a result, in the equivalent circuit diagram shown in FIG. 17, the inductor components L11, L12 are not present, or the inductor components significantly decrease. Therefore, the characteristic impedance of a portion of the signal line 602 overlapping the bridge portions 608 is such that the capacitor component C10 is dominant, and the characteristic impedance of that portion is lower than a predetermined characteristic impedance. In view of the foregoing, in the flexible substrate 600 described in Japanese Laid-Open Publication No. 2007-123740, it is necessary to increase the distance between the signal line 602 and the ground layer 604. Thus, it is difficult to reduce the thickness of the flexible substrate 600.